January 1 – December 31, 2020 February 2021

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Animal and Plant

Health Inspection

Service

Veterinary

Services

2020 Summary of West Nile Virus

Equine Cases in the United States

This report is a descriptive summary of U.S. equine West Nile virus (WNV) cases reported to the United

States Department of Agriculture (USDA) in 2020. The USDA Animal and Plant Health Inspection Service

(APHIS) Veterinary Services (VS) branch collaborates with the U.S. Centers for Disease Control and

Prevention (CDC) and State veterinary and public health officials to facilitate communication about

arbovirus disease cases in horses and confirm equine cases in each State. CDC collects arbovirus case

information using its ArboNET reporting system, an electronic surveillance and reporting system for

tracking and reporting arboviral activity in humans and animals. During the transmission season, VS

disseminates the equine case information to State animal health officials for confirmation and posts the

number of confirmed cases to the USDA WNV information web site

(https://www.aphis.usda.gov/aphis/ourfocus/animalhealth/animal-disease-information/equine). This

site also provides specific case definitions. In this document, ‘reported cases’ of WNV may include

presumptive positive and confirmed positive cases.

In 2020, 71 equine cases of WNV were reported from 19 States. Figure 1 shows the annual number of

equine WNV cases reported in the United States from 2007 through 2020. Figure 2 shows the number of

equine WNV cases in each State in 2020, and Table 1 provides the number of cases per county or parish

in those States in 2020.

Equine Cases of WNV

627

600

500

400

300

200

100

0

468

179

276

125

87

377

141

225

380

307

493

90 71

2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020

WNV Cases

Figure 1. Reported U.S. equine cases of West Nile virus, 2007-2020
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2020 Summary of West Nile Virus Equine Cases in the United States

Figure 2. Distribution of reported equine WNV cases in 2020 (71 total cases)

Table 1. Number of reported equine WNV cases by State and county, 2020

State County West Nile Virus

Reported Cases

Arizona Maricopa County 1

Pinal County 1

Yavapai County 1

Total cases for Arizona 3

Arkansas Newton County 1

Total cases for Arkansas 1

California Amador County 2

Butte County 1

Glenn County 1

Kings County 1

Merced County 1

Modoc County 1

USDA-APHIS-VS 2
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2020 Summary of West Nile Virus Equine Cases in the United States

State County West Nile Virus

Reported Cases

Nevada County 1

Riverside County 2

Sacramento County 1

San Bernardino County 1

San Joaquin County 4

Solano County 1

Stanislaus County 3

Total cases for California 20

Colorado Jefferson County 1

Total cases for Colorado 1

Florida Broward County 1

Collier County 1

Hendry County 1

Hillsborough County 1

Holmes County 1

Lake County 1

Lee County 3

Manatee County 1

Okaloosa County 1

Okeechobee County 1

Palm Beach County 2

Total cases for Florida 14

Idaho Canyon County 1

Gooding County 1

Total cases for Idaho 2

Indiana Delaware County 1

Lagrange County 1

La Porte County 1

Total cases for Indiana 3

Kansas Douglas County 1

Johnson County 1

Sedgwick County 1

Total cases for Kansas 3

Kentucky Madison County 1

Washington County 1

Total cases for Kentucky 2

Louisiana St. Tammany Parish 3

USDA-APHIS-VS 3
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2020 Summary of West Nile Virus Equine Cases in the United States

State County West Nile Virus

Reported Cases

Tangipahoa Parish 1

Vermilion Parish 1

Winn Parish 1

Total cases for Louisiana 6

Missouri Vernon County 1

Total cases for Missouri 1

Montana Rosebud County 1

Total cases for Montana 1

Nebraska Holt County 1

Wayne County 1

Total cases for Nebraska 2

North Carolina Camden County 1

Total for North Carolina 1

North Dakota Mountrail County 1

Total cases for North

Dakota 1

South Carolina Charleston County 1

Total cases for South Carolina 1

South Dakota Pennington County 1

Total cases for South Dakota 1

Texas Erath County 1

Hardeman County 1

Jones County 1

Taylor County 1

Total cases for Texas 4

Utah Utah County 2

Weber County 2

Total cases for Utah 4

Total Reported West Nile Virus Equine Cases for the United States 71

WNV can be a deadly disease in equids, with mortality occurring in roughly 33 percent of all infected

animals. Furthermore, up to 40 percent of affected equids may still exhibit neurological signs as long as

6 months after diagnosis.1 This susceptibility to the virus makes equids a recognized WNV sentinel

species for the human population. The main public health purpose of confirming and reporting equine

WNV cases is to inform human surveillance efforts. Because equine commercial vaccines are available

and convey good protection against WNV infection, reported cases of WNV in equids tend to be in

unvaccinated or under-vaccinated animals.

1. “Core Vaccination Guidelines: West Nile Virus.” American Association of Equine Practitioners,

https://aaep.org/guidelines/vaccination-guidelines/core-vaccination-guidelines/west-nile-virus. Accessed 9 April 2021.

USDA-APHIS-VS 4